linux-kernel - Re: [PATCH 7/7] random: add backtracking protection to the CRNG

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Date:	Sun, 26 Jun 2016 20:47:53 +0200
From:	Pavel Machek <pavel@....cz>
To:	Theodore Ts'o <tytso@....edu>
Cc:	Linux Kernel Developers List <linux-kernel@...r.kernel.org>,
	linux-crypto@...r.kernel.org, smueller@...onox.de,
	herbert@...dor.apana.org.au, andi@...stfloor.org,
	sandyinchina@...il.com, jsd@...n.com, hpa@...or.com
Subject: Re: [PATCH 7/7] random: add backtracking protection to the CRNG

On Mon 2016-06-13 11:48:39, Theodore Ts'o wrote:
> Signed-off-by: Theodore Ts'o <tytso@....edu>
> ---
>  drivers/char/random.c | 54 ++++++++++++++++++++++++++++++++++++++++++++++-----
>  1 file changed, 49 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/char/random.c b/drivers/char/random.c
> index d640865..963a6a9 100644
> --- a/drivers/char/random.c
> +++ b/drivers/char/random.c
> @@ -436,7 +436,8 @@ static int crng_init = 0;
>  #define crng_ready() (likely(crng_init > 0))
>  static void _extract_crng(struct crng_state *crng,
>  			  __u8 out[CHACHA20_BLOCK_SIZE]);
> -static void extract_crng(__u8 out[CHACHA20_BLOCK_SIZE]);
> +static void _crng_backtrack_protect(struct crng_state *crng,
> +				    __u8 tmp[CHACHA20_BLOCK_SIZE], int used);
>  static void process_random_ready_list(void);

You can do u8 and u32 in the code, we know we are in kernel.

> +/*
> + * Use the leftover bytes from the CRNG block output (if there is
> + * enough) to mutate the CRNG key to provide backtracking protection.
> + */
> +static void _crng_backtrack_protect(struct crng_state *crng,
> +				    __u8 tmp[CHACHA20_BLOCK_SIZE], int used)
> +{
> +	unsigned long	flags;
> +	__u32		*s, *d;
> +	int		i;
> +
> +	used = round_up(used, sizeof(__u32));
> +	if (used + CHACHA20_KEY_SIZE > CHACHA20_BLOCK_SIZE) {
> +		extract_crng(tmp);
> +		used = 0;
> +	}
> +	spin_lock_irqsave(&crng->lock, flags);
> +	s = (__u32 *) &tmp[used];
> +	d = &crng->state[4];
> +	for (i=0; i < 8; i++)
> +		*d++ ^= *s++;
> +	spin_unlock_irqrestore(&crng->lock, flags);
> +}

You are basically trying to turn CRNG into one way hash function here,
right? Do you have any explanation that it has the required
properties?

Thanks,
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html